Head Gasket, A noob question Options

[Dunners]

Hey guy's,
I did a bit of a search and couldn't find anything that really answered my question. Could someone explain to me exactly what a head gasket is, what it does, Where it is and exactly how to get to it and maybe replace it with a stronger one. Is it possible to get a stronger stock one off a better motor and fits the 5sfe? Also today i noticed coolant dripping down onto my splash guard, i think it's coming from the top of the overflow where there's a little hole. It hasn't done this before, does anyone know what would cause this. Thanks for all your help guy's.
Dunners.